In numerous operations dealing with gases, liquids and other flowable materials it is necessary to remove contaminants and, of course, one way of removing them is by filtering. A problem in filtering is that filter screens get clogged and it is necessary to provide a fresh filter member in order to continue the operation. While there are many operations where it is entirely satisfactory to shut things down for a change of filter, there are many where it is impossible or at least unacceptably costly to do so. Many industrial and commercial operations require filtering under difficult conditions such that changing a filter element such as a filter screen is an awkward, time consuming and difficult task complicated by the fact that a filter element rapidly changed may leak or be torn and not fitted properly, or a seal at the filter may not be properly seated or tightly sealed. Perhaps even more to the point, in many situations variations in flow that may result from plugging or changing of filters can cause serious impairment of product quality or uniformity.
One of the most difficult examples of problems in filtering is in connection with extrusion of plastic. Ordinarily plastic material in flowable state is caused to flow to a location where uniformity of product is important. It is usually extruded hot and under pressure, and for purposes where continuity of flow is critical. This flow may, for example, be directed toward an extrusion die, to an injection mold to or from a melt pump, to a filament-forming die, or as may be required by a polymer producer, to a storage or shipping container. In this description of the invention, the plastics industry uses will generally be expressed in terms of extrusion and in terms of dies and molds, but it is to be realized that other flow usage of material in flowable form is included.
The present invention deals generally with filtering operations and, in a specific instance, with filtering of plastics which is one of the arts where filtering is beset with problems. Even new plastic material shipped directly from the manufacturer is likely to have some such contaminants, either in the plastic itself or picked up from the container in which the plastic is shipped. In particular, such plastic may have strings or "globs" or more highly polymerized material which will impair the final product. In any event, however, successful and economic operations of extrusion devices may require the reclaiming and recycling of scrap plastic. Such scrap inevitably comes contaminated, picking up solid particles which it carries with it into the mold. If these solid particles are allowed to get into a die or mold then, at best, they cause the formation of faulty products and at worst they will clog or damage the apparatus.
As with contaminants in many materials, this problem has been with the plastics industry for many, many years and is generally handled by filtering the molten plastic on as nearly as possible a continuous basis. Such filtering when it operates well does an effective job of removing solid contaminants from the plastic. Filtering the molten plastic, however, has its problems, some of which are specifically related to the nature of plastic materials. Plastic is generally injected or pumped into a mold, die or other apparatus at a relatively high pressure such as for example, 1,000 to 5,000 pounds per square inch, and is necessarily injected at a sufficiently high temperature so that the plastic flows quite well. Depending on the specific plastic being employed the operating temperature is likely to be in the general order of 200.degree.F or 300.degree.F or higher. The filter medium, therefore, must be strong and resistant to elevated temperatures. This, however is only the start of the problem. In many filtering operations a relatively large quantity of solid contaminants may be filtered out of the plastic in a very short time so that the filter members can become completely or partially clogged in a very few minutes, stopping or reducing the rate of flow or changing the flow pressure, necessitating a change of the filter. It is entirely unsatisfactory to employ a filter which must be disassembled each time the filter medium is plugged, and accordingly it is important to find a suitable filter which can be changed and either replaced or cleaned without shutting down or slowing down the operation. The filtering systems and structures which have been employed in the plastics industry to solve this problem have been many different shapes, styles and kinds annd have had as their principal common characteristic the fact that they are not satisfactory.
It costs many dollars to shut down an operation for the time necessary to change a part such as a filter screen, so some way is needed to change such filter screens instantaneously or continuously. The most common system, and one which was developed many, many years ago and is still in use for filtering of plastic, employs a slidable filter element having two filter sections. The mold or die is operated with one of these filter sections in position to filter contaminants out of the flowing plastics. When it becomes sufficiently clogged so that cleaning is necessary, the other filter element is moved into position, leaving the first filter section extending outside the extrusion mold or pipe line so that it can be cleaned or replaced without stopping operation of the machine. One such system was described in Cowen U.S. Pat. No. 642,814 dated Feb. 6, 1900. As was said, this system is in one form or another still in use and does work to permit filtering of the plastic and to permit changing of filters. When such a filter becomes partially or completely clogged, the slide plate is forceably moved to its opposite position and the filter is replaced or cleaned. At best this system is not ideal because the filter generally becomes gradually more and more plugged with consequent reduction of flow, until finally the impaired flow can no longer be tolerated. The result is that the operation seldom runs at good efficiency. Other systems have since been proposed in which a succession of filter elements can be moved through a filter zone at such filter elements become clogged. More recent answers to the problems have been attempted and are illustrated in patents such as Bounin U.S. Pat. No. 3,084,987 and one or more patents to Kalman such as, for example, U.S. Pat. No. 3,471,017. With all of the proposed answers to the problems some degree of success has been achieved, but in every case the success is limited. In general, however, it appears to be desirable to employ a filter having a large number of filter sections so that filtering operation can be carried out for a long time without interruption. The experience with such filter systems in general is that in some of them there is a danger of tearing or distorting the filter when it is repeatedly moved step by step through the filter zone, in most of them there is an extremely serious danger that the filter seals will not be tight, and in many of them there is a gradual drop-off in filter flow so that process or product non-uniformity continues to be a serious problem. With all the improvement in the art through the years there has as yet been no solution to the problem of maintaining a uniformly clean, tight filter.
The filter system of the present invention has been specifically designed and engineered for the plastics industry, but it is of course useful in other industries and applications where similar requirements exist. For example, certain filtering operations in the rubber industry and in other industries can be handled by this filtering system.